During polyolefin production process, gas (which is generally called as vent gas) generated from reactor, flash tank, degassing column and other devices contains large amounts of inert gases (such as N2 and saturated hydrocarbons), unreacted olefin monomer, and other kinds of gas, in which hydrocarbons have a very high economic value, and N2 can be recovered and reused after being purified (for example, N2 can serve as a purge gas of a degassing column). In traditional technology, the vent gas is generally recovered by a compression-condensation method. Since different components have different boiling points, through reducing a temperature of the vent gas, some gas can be liquefied and separated. However, if a high recovery is required, energy consumption and an investment of the compression-condensation method will become relatively high, which cannot meet the improved requirement. Therefore, other methods with a higher separation factor and a higher efficiency are developed, such as a membrane separation method and a pressure swing adsorption method (US patents U.S. Pat. No. 6,706,857 and U.S. Pat. No. 5,769,927). These methods should be used in combination with the compression-condensation method, because the product obtained after separation is in vapor-phase, and purity and pressure thereof cannot meet the requirement of direct reuse. In addition, people have made improvements on the method in which liquefaction is realized through temperature reduction, and the most prominent improvement is the realization of cryogenic technology through turbo expansion or throttle expansion (CN201310444283). Moreover, it is discovered that, different combinations of these methods can bring about different effects. That is, from the aspect of separation engineering, the design of separation sequence is very important. Different separation sequences can lead to rather different recoveries, product purities, energy consumptions, and investments. It is regrettable that, in the prior art, different separation technologies are only simply combined, while the optimized separation sequence is not developed, which leads to various problems.
Chinese patent CN200910038599.7 discloses a method for completely recovering vent gas in polyethylene plant, and a separation sequence of this method is compression-condensation separation, hydrocarbon selective membrane separation, and pressure swing adsorption separation. However, in this method, heavy hydrocarbons and light hydrocarbons cannot be separated, and the hydrocarbons obtained therein have a low concentration; N2 and H2 cannot be separated, and thus when a concentration of H2 is relatively high, N2 cannot be reused as a purge gas of a degassing column; and off-gas after pressure swing adsorption separation still has a relatively high pressure, which cannot be effectively utilized, and thus the energy consumption of the method is relatively high.
Chinese patent CN201310444283 discloses a system and a method for recovering emissions generated from an olefin polymerization process. According to this method, the separation sequence is compression-condensation separation, hydrogen selective membrane separation, and cryogenic separation. The patent mentioned that H2 is separated from the vent gas by a hydrogen selective membrane to improve a purity of N2, so that N2 can be recycled and reused as a purge gas of a degassing column. Since the membrane separation step is arranged before the cryogenic separation step, when H2 permeates the membrane, a large amount of hydrocarbon components permeate the membrane at the same time. Consequently, recovery of hydrocarbons is reduced. Moreover, the following cryogenic separation step is essentially one gas-liquid equilibrium separation step. Due to the limitation of an equilibrium separation temperature, the N2 which is separated from a top of a separator still contains a small amount of hydrocarbon which cannot be treated, and thus a purity of N2 obtained therein is not high.
US patent U.S. Pat. No. 5,769,927 discloses a monomer recovery process, and a separation sequence of this process is compression-condensation separation and membrane separation. Due to operating limit of a hydrocarbon selective membrane module, a condensation temperature of this process is relatively high (about −30° C.). Some heavy hydrocarbons can be separated through condensation separation, but a large amount of light hydrocarbons need to be separated by the following membrane separation step. According to this process, the light hydrocarbons, which are separated by the membrane, are all returned to a compressor, and the only outlet of hydrocarbons is arranged at a separator in the compression-condensation separation step. This separation sequence will result in a large gas circulation amount returning to the compressor after the membrane separation step. Moreover, the content of hydrocarbons in the gas needs to be accumulated to a high level before the compression-condensation separation step, the hydrocarbons in the original feed stream can be completely condensed here. Consequently, energy consumption of the compressor is significantly increased. In addition, heavy hydrocarbons and light hydrocarbons are not separated by this process, and thus cannot be reused respectively. N2 and H2 are not separated either, and thus N2 is not suitable to serve as a purge gas of a degassing column, since the accumulation of H2 will bring about safety risk to the operation of the degassing column.
Chinese patent CN201510294040.6 discloses a method for recovering vent gas according to gas phase process polyolefin method, and a separation sequence of the method is compression-condensation separation, ordinary cryogenic separation, hydrocarbon selective membrane separation, hydrogen selective membrane separation, and cryogenic separation. In this method, a hydrocarbon selective membrane separation step is added. However, with this separation sequence design, a high amount of hydrocarbons need to be treated by hydrocarbon selective membrane. After the ordinary cryogenic separation step, the gas still contains a large amount of hydrocarbons. A large amount of gas is returned to the compressor after the hydrocarbon selective membrane separation, and energy consumption of the compressor is rather high. In addition, after the hydrocarbon selective membrane separation and the hydrogen selective membrane separation, the gas enters the cryogenic separation step, and N2 with a high purity can be obtained only at a lower separation temperature due to the limitation of the equilibrium temperature. As a result, the equipment investment and operating energy consumption of the cryogenic separation step in this method are higher than the cryogenic separation step in the aforesaid patents. Moreover, the patent points out that, in the ordinary cryogenic separation step, when a concentration of hydrocarbons in the vent gas is not high, an external refrigerant should be added. It is regrettable that, the concentration of hydrocarbons in the vent gas of polyolefin production is generally low, and thus a low-temperature external refrigerant needs to be added. Due to the limitation of an operating temperature of membrane module, the temperature of the ordinary cryogenic separation step cannot be arranged too low, and the recovery of hydrocarbons in the ordinary cryogenic separation step is not high. Therefore, the main role of the following hydrocarbon selective membrane separation step is separating hydrocarbons rather than purifying N2. Although the hydrogen selective membrane separation step is arranged later, there will be a certain amount of hydrocarbons permeating through the membrane together with H2 since hydrocarbons are not completely separated in the hydrocarbon selective membrane separation step, which will result in the loss of hydrocarbons.
US patent U.S. Pat. No. 6,706,857 discloses a method for recovery of olefin monomers, and the separation sequence of the method is cryogenic separation and pressure swing adsorption separation. According to this method, the pressure swing adsorption separation is used in the vent gas of the polyolefin production recovering field, and the adsorbent is defined. However, in the cryogenic separation step, an additional external refrigerant is used, which is energy consuming and is not economical. According to the description of the patent, the pressure swing adsorption separation step mainly plays the role of separating hydrocarbons. As a result, the amount of gas which is returned to the compressor is rather large, and energy consumption of the compressor will be increased. In the examples of the patent, the vent gas does not contain heavy hydrocarbon. However, actually, the vent gas of polyolefin production process (includes polyethylene production and polypropylene production) mostly comprises heavy hydrocarbon components at present. Therefore, according to this method, heavy hydrocarbons and light hydrocarbons cannot be separated. In addition, H2 and N2 cannot be separated either, and N2 with a high purity cannot be obtained.
US patent U.S. Pat. No. 574,350 discloses a method for recovering hydrocarbons from vent gas in polyolefin plant, and the separation sequence of this method is compression-condensation separation, oil absorption separation, and cryogenic separation. The oil absorption separation step is essentially a gas-liquid equilibrium separation step. Therefore, only high efficient absorbent is used, can the hydrocarbons in the vent gas be effectively absorbed. The high efficient absorbent should have two features, first, it has a good absorption effect on hydrocarbons, and second, the absorbent is a nonvolatile substance itself. However, in the patent, this kind of absorbent is not provided, while a heavy hydrocarbon and the like is used as the absorbent. Therefore, the oil absorption effect is unsatisfactory. During the cryogenic separation step, similar to the aforesaid patent CN201310444283, the N2-rich gas from a top of a separator still contains a small amount of hydrocarbon which cannot be further treated, and thus a purity of N2 obtained therein is not high. In addition, according to this method, the H2 in the N2-rich gas is not separated.
In view of the technical problem in the prior art, the present disclosure provides a process for recovering valuables from vent gas in polyolefin production.